Electroluminescent display device



Sept. 12, 1961 E. R. BOWERMAN, JR., EI'AL ELECTROLUMINESCEINT DISPLAY DEVICE Filed April 24, 1959 I NVENTORS EDWIN R0) BUWERM/M/JR.

ALBERT J- MAR/(0 ATTORNEY United States Patent 2,999,958 ELECTROLUMINESCENT DISPLAY DEVICE Edwin Roy Bowerman, Jr., Whitestoue, and Albert Marko, Deer Park, N.Y., assignors to Sylvania Electric Products Inc., a corporation of Delaware Filed Apr. 24, 1959, Ser. No. 808,745

2 Claims. (Cl. 313-108) Our invention is directed to electroluminescent display devices.

Electroluminescent phosphors are phosphors which emit light in the presence of an electric field. These phosphors can be formed into electroluminescent layers or films, as for example by dispersing phosphor particles in a solid dielectric.

One type of electroluminescent device, known as a cross grid electroluminescent panel, comprises an electroluminescent layer, one surface of which is coated with a plurality of separate horizontal electrodes, the opposite surface of the layer being coated with a plurality of separate vertical electrodes. When a suitable voltage is applied between any horizontal electrode-vertical electrode pair, that portion of the electroluminescent layer subtended between the electrodes forming this pair (termed a cell) will luminesce. Further, when the applied voltage is switched from one electrode pair to another, each cell can be successively energized in turn, thus producing an effect analogous to a scanning operation in a cathode ray tube. Gradations in the light intensity can be produced by suitably modulating the amplitude of the applied voltage.

We have invented a new type of cross grid electroluminescent panel which provides much sharper image definition than heretofore obtainable. Further, the electrical power consumption of our panel is sharply reduced compared to that hitherto required.

In accordance with the principles of our invention, a first set of parallel separate electrodes is applied to one surface of a glass plate. An electroluminescent layer is applied over the first set electrodes. A second set of parallel separate electrodes, oriented at an angle other than zero with respect to the first set electrodes (for example an angle of 90), is applied over the electroluminescent layer.

Both the first set of electrodes and the second set of electrodes are optically opaque. Both the first set elec trodes and the second set electrodes are formed from metal. The electrodes of both sets have equal and extremely narrow widths. More particularly, the individual electrode widths and the thickness of the electroluminescent layer are dimensionally comparable; i.e. these two dimensions are of the same order of magnitude.

As a consequence of this construction, the electrical resistance of the two sets of electrodes is much lower than hitherto obtainable, and the power consumption is reduced accordingly. Further, the number of electrodes per linear inch of panel can be sharply increased, with a consequent increase in image definition.

Since the electrodes are extremely narrow, application of the switching or modulating voltage produces fringe electric fields, whereby sufiicient light is emitted from any cell despite the use of optically opaque el ctrodes.

An illustrative embodiment of our invention will now be described with reference to the accompanying figure.

Referring now to the figure, there is shown a sheet of glass 10. A first set of vertical separate electrodes 12 is applied to one surface of the sheet 10. These electrodes 12 are formed from a metal such as gold or platinum and are optically opaque. Electrodes 12 are extremely narrow, for example, having a width of mils or less. v

ice

A thin electroluminescent layer 14, composed of electroluminescent phosphor particles embedded in a ceramic or plastic dielectric, is applied over the first set of electrodes. Typically, layer 14 has a thickness of l-3 mils.

A second set of horizontal electrodes 16 is applied over the electroluminescent layer 14. Electrodes 16 are optically opaque and are formed from a metal such as aluminum. The widths of electrodes 16 and 14 are substantially identical.

When a voltage is applied between any horizontal electrode-vertical electrode pair, the resultant electric field is not uniformly normal to the electrodes of this pair, but rather is curved, thus permitting light to be emitted around each opaque electrode. (The portion light output from this fringe field action increases significantly when the electrodes are sufliciently narrow; i.e. when the electrode width and the thickness of the electroluminescent layer are of the same order of magnitude.)

The use of the metal electrodes previously described sharply reduces the power requirements of the device of the figure, since the electrical resistance of these electrodes is much smaller (by a factor of 10 or even as compared to the resistance of the transparent stannous chloride electrodes conventionally employed. (Stannous chloride is effectively a semiconductor, not a metal conductor.)

Further, the electrode ruling can be as high as 50 electrodes per linear inch (as compared to conventional ruling of 16 electrodes per inch) with consequent increase in image definition.

In forming the device shown in the figure, the glass plate is first coated with a liquid gold luster solution as, for example Hanovia Liquid Bright Gold. The coated plate is then fired in air, for example to a temperature of 550 C. to produce an adherent gold film. A photoresist lacquer is applied over the gold film, and a pattern of vertical electrodes is printed thereon. The pattern is developed. Thereafter, the excess gold is etched away.

After the first set of electrodes have been so produced, a mixture of electroluminescent phosphor and ceramic frit is applied thereover as, for example by spraying. The structure is then fired to produced an adherent electroluminescent enamel layer. An aluminum film is then evaporated onto the surface of the electroluminescent layer. A pattern of horizontal electrodes is printed thereon, and the excess aluminum is removed, in the same manner as described above for the gold electrodes.

We have found that the first set of electrodes must be formed from a metal which, in addition to the low electrical resistivity, must be inert to the above described procedure; if an inert metal is not used, the electrodes enter into chemical reaction, and the resultant device is unusable. Further, the material must not crack during processing. Gold and platinum are two metals we have found which Will satisfy these requirements.

Further, we have found that if the second set of electrodes is prepared by the above process, gold or platinum cannot be used since these metals tend to go into the electroluminescent layer rather than remaining as desired on the surface of the layer. However, evaporated metal coatings such as aluminum coat the electroluminescent layer in an acceptable manner.

What is claimed is:

1. An electroluminescent device comprising an electroluminescent layer; a first set of parallel separate optically opaque electrodes extending in a first direction and secured to one surface of said layer, said first set of electrodes being composed of an inert metal selected from the class consisting of gold and platinum; and a second set of parallel separate optically opaque electrodes extending in a second and non-parallel direction and secured to the other surface of said layer, said second set electrodes being composed of aluminum, the electrodes in at least one ofsaid' sets of electrodes having widths substantially equal to the thickness of said electroluminescent layer, the thickness of said electroluminescent layer being between l and 3 mils;

2. An electroluminescent display device comprising anelectroluminescent layer; a first set ofparallel separate optically opaque electrodes extending in a first direction and secured to one surface of said layer, said first set electrodes beingcomposed of an inert metal selected from the class consisting of gold and platinum; and a second set of parallel separate optically opaque electrodes extending in a second and non-parallel direction and; secured to the other surface of said layer, said second set electrodes being composed of aluminum, said electroluminescentlayer having a thickness fallingwithin the approximate range 1-3 mils, the widths of the electrodes of said first and second sets being substantially equal to 5 the thickness of said electroluminescent layer.

References. Cited inthefil'e of this patent UNITED STATES-i PATENTS.

10 2,875,380 Toulon- Feb. 24', 1959 2,877,371 Orthuber' Mar. 10, 1959 2,883,582 Hanlet Apr. 21, 1959 2,925,532 Larach ..Feb; 16, 19.60

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1. AN ELECTROLUMINESCENT DEVICE COMPRISING AN ELECTROLUMINESCENT LAYER; A FIRST SET OF PARALLEL SEPARATE OPTICALLY OPAQUE ELECTRODES EXTENDING IN A FIRST DIRECTION AND SECURED TO ONE SURFACE OF SAID LAYER, SAID FIRST SET OF ELECTRODES BEING COMPOSED OF AN INERT METAL SELECTED FROM THE CLASS CONSISTING OF GOLD AND PLATINUM; AND A SECOND SET OF PARALLEL SEPARATE OPTICALLY OPAQUE ELECTRODES EXTENDING IN A SECOND AND NON-PARALLEL DIRECTION AND SECURED TO THE OTHER SURFACE OF SAID LAYER, SAID SECOND SET ELECTRODES BEING COMPOSED OF ALUMINUM, THE ELECTRODES IN AT LEAST ONE OF SAID SETS OF ELECTRODES HAVING WIDTHS SUBSTANTIALLY EQUAL TO THE THICKNESS OF SAID ELECTROLUMINESCENT LAYER, THE THICKNESS OF SAID ELECTROLUMINESCENT LAYER BEING BETWEEN 1 AND 3 MILS. 